Recent studies by Eisenberg and Felsenfeld on poly A and by Inners and Felsenfeld on poly U have indicated that the local conformations of polyribonucleotide chains are fairly restricted even in the absence of base-stacking interactions. Current work in our laboratory indicates that natural nucleic acids are similarly limited in their range of local conformations. The proposed study is concerned with the comparison of the unperturbed dimensions of various single-stranded DNAs with those observed for the single-stranded synthetic homopolymers. Light scattering photometry and viscometry are being employed to determine molecular weights and radii of gyration. Other physical techniques such as circular dichroism measurements and sedimentation in the ultracentrifuge are being used to assess secondary structure and molecular weight distributions. Experiments with single-stranded calf thymus DNA fractionated by reversible phase separation have shown that ideal solvent conditions can be realized in dilute formaldehyde solutions at high ionic strength and in alkaline ethanol-water solvent systems. Other solvents containing denaturing agents such as formamide and ethylene glycol will be tested as possible theta solvents.